Magnetically actuated motion control devices such as magnetically controlled dampers or struts provide motion control, e.g., damping that is controlled by the magnitude of an applied magnetic field. Much of the work in the area of magnetically controlled dampers has focused on either electrorheological (ER) or magnetorheological (MR) dampers. The principle underlying both of these types of damping devices is that particular fluids change viscosity in proportion to an applied electric or magnetic field. Thus, the damping force achievable with the fluid can be controlled by controlling the applied field. Examples of ER and MR dampers are discussed in U.S. Pat. Nos. 5,018,606 and 5,384,330, respectively.
MR fluids have high yield strengths and viscosities, and therefore are capable of generating greater damping forces than ER fluids. In addition, MR fluids are activated by easily produced magnetic fields with simple low voltage electromagnetic coils. As a result, dampers employing MR fluids have become preferred over ER dampers.
Because ER and MR fluid dampers still involve fluid damping, the dampers must be manufactured with precise valving and seals. In particular, such dampers typically require a dynamic seal and a compliant containment member which are not particularly easy to manufacture and assemble. Further, the fluid type dampers can have significant “off-state” forces which can further complicate manufacture and assembly. Off-state forces refer to those forces at work in the damper when the damper is not energized.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.